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REVIEW
The effects of green tea on weight loss and weight
maintenance: a meta-analysis
R Hursel
1
, W Viechtbauer
2
and MS Westerterp-Plantenga
1
1
Department of Human Biology, Nutrition and Toxicology Research Institute Maastricht (NUTRIM) Maastricht University,
Maastricht, The Netherlands and
2
Department of Methodology and Statistics, Maastricht University, Maastricht,
The Netherlands
Introduction. Different outcomes of the effect of green tea on weight loss (WL) and weight maintenance (WM) have been
reported in studies with subjects differing in ethnicity and habitual caffeine intake.
Purpose. To elucidate by meta-analysis whether green tea indeed has a function in body weight regulation.
Methods. English-language studies about WL and WM after green tea supplementation were identified through PubMed and
based on the references from retrieved articles. Out of the 49 studies initially identified, a total of 11 articles fitted the inclusion
criteria and provided useful information for the meta-analysis. Effect sizes (mean weight change in treatment versus control
group) were computed and aggregated based on a random-effects model. The influence of several moderators on the effect
sizes was examined.
Results.Catechins significantly decreased body weight and significantly maintained body weight after a period of WL
(^
m¼1.31 kg; Po0.001). Inhibition of this effect by high habitual caffeine intake (4300 mg per day) failed to reach significance
(^
m¼0.27 kg for high and ^
m¼1.60 kg for low habitual caffeine intake; P¼0.09). Also, the seemingly smaller effect of
catechins in Caucasian (^
m¼0.82 kg) subjects compared with Asians ( ^
m¼–1.51 kg; P¼0.37) did not reach significance.
Interaction of ethnicity and caffeine intake was a significant moderator (P¼0.04).
Conclusions.Catechins or an epigallocatechin gallate (EGCG)–caffeine mixture have a small positive effect on WL and WM. The
results suggest that habitual caffeine intake and ethnicity may be moderators, as they may influence the effect of catechins.
International Journal of Obesity (2009) 33, 956–961; doi:10.1038/ijo.2009.135; published online 14 July 2009
Keywords: green tea; caffeine; weight loss; weight maintenance; ethnicity
Introduction
Green tea is the most widely consumed beverage next to
water in Asian countries. Moreover, people in Asia have been
aware of the beneficial effects of green tea on health for
already hundreds of years.
1
On the other hand, only recently
is green tea gaining interest in the Westernized world, where
mainly black tea is drunken. Green tea is made from the
fresh leaves of Camellia sinensis, which are processed rapidly
by means of steam to prevent fermentation. Black tea is
made from the same plant as green tea but the leaves endure
an extra enzymatic oxidation step during the processing.
1
The polyphenols in green tea such as epigallocatechin gallate
(EGCG), epigallocatechin, and epicatechin gallate, which are
responsible for its beneficial effects (such as the anti-oxidant
potential and anti-mutagenic capacity) are converted into
thearubigins and theaflavins in black tea.
2
Although black
tea still can improve one’s health, the conversion attenuates
the effects reported after intake of green tea.
3–5
Since the 1990s, green tea is also seen as a natural herb
that can enhance energy expenditure and fat oxidation
and thereby induce weight loss (WL).
6,7
The fact that
an EGCG–caffeine mixture stimulates energy expenditure
cannot be completely attributed to its caffeine content
because the thermogenic effect of an EGCG–caffeine mixture
is greater than that of an equivalent amount of caffeine.
8
Twenty-four hour studies showed a significant increase in
energy expenditure and fat oxidation.
8,9
From these short-
term studies, studies over the longer term originated in
which the effect from an EGCG–caffeine mixture on WL was
studied and soon studies concerning the effect on weight
maintenance (WM) followed.
Almost all of the studies conducted with Asian subjects
have shown positive results about the anti-obesity effects of
Received 30 January 2009; revised 18 May 2009; accepted 2 June 2009;
published online 14 July 2009
Correspondence: R Hursel, Department of Human Biology, Maastricht
University, Universiteitssingel 50, P.O. Box 616, Maastricht, 6200 MD,
The Netherlands.
E-mail: rick.hursel@hb.unimaas.nl
International Journal of Obesity (2009) 33, 956 –961
&
2009 Macmillan Publishers Limited All rights reserved 0307-0565/09
$
32.00
www.nature.com/ijo
catechins.
10–17
On the other hand, studies with Caucasian
subjects reported mixed results.
18–20
Moreover, the
amount of regular caffeine intake may also influence the
effectiveness of catechins. In a study from Westerterp-
Plantenga et al.,
20
it was shown that a regular caffeine intake
of 4300 mg per day in Caucasian subjects diminished the
effects of an EGCG–caffeine mixture on WM, in comparison
with subjects that had a regular caffeine intake of o300 mg
per day.
Hence, the aim of this study was to do a systematic review
and meta-analysis of all the available long-term studies that
have examined the effects of EGCG on WL and WM, to
establish whether catechins contribute to the regulation of
body weight. Possible moderators, such as regular caffeine
intake and ethnicity, are included in the meta-analysis, to
investigate whether they have any impact on the effects
of an EGCG–caffeine mixture on body weight.
Methods
Identification of relevant studies
PubMed was searched for English-language studies that
reported the long-term effects from catechins on WL and
WM. The databases were searched up to July 2008 with
the following keywords: green tea, body weight, WL, WM,
catechins, epigallocatechin gallate or EGCG. References from
the retrieved articles were examined to identify possible
additional articles.
Inclusion and exclusion criteria
A study was valid for inclusion in the review if (a) the study’s
participants were randomly assigned to conditions, (b) the
results allowed a direct comparison of the effects of catechins
(either in the form of catechins versus no catechins or high
dosage versus low dosage catechins) on either WL or WM,
(c) subjects were kept blind to the condition assigned, and
(d) the study lasted for at least 12 weeks. An EGCG–caffeine
mixture tea could either be administered as regular tea or
as capsules. An important exclusion criterion was the use
of any tea other than an EGCG–caffeine mixture (such as
oolong tea).
Data abstraction
Various characteristics were extracted from the original
reports using a standardized data extraction form. We recor-
ded the study’s author(s), year of publication, the duration of
the treatment phase (in weeks), whether the study used a WL
or WM design, and the age range, ethnicity, average body
mass index, gender, and average regular caffeine intake
(in mg per day) of the participants in the study. Moreover, for
each condition, we recorded the number of participants and
the dosage of green tea administered (in mg per day).
For each condition, we calculated or extracted the mean
weight change (in kilograms) between the post-treatment
and the baseline assessment. For WL studies, the baseline
corresponds to the point just before the commencement of
the WL phase, whereas for WM studies, the baseline
corresponds to the point before the commencement of the
WM phase. The post-treatment mean was always based on
the time point immediately following the intervention.
Regardless of the study design, negative values for the mean
change, therefore, indicate WL, a value of zero indicates no
change, and positive values indicate an increase in the body
mass on average. The effect size measure for the meta-
analysis was then the difference between the mean change
value for the treatment minus the mean change value for the
control group. Negative values for the observed effect size,
therefore, indicate more WL (or less weight gain) on average
in the treatment compared with the control group and,
therefore, a positive treatment effect of the EGCG–caffeine
mixture on WL/WM.
The following three variables were a priori chosen for
examination as potential moderators: the ethnicity of the
participants (either Asian or Caucasian), the average regular
caffeine intake of the participants in the study (either low
or high, with 300 mg per day being the cutoff), and the
difference in dosage of the EGCG–caffeine mixture adminis-
tered in the treatment and the control group.
Statistical analysis
The effect size estimates were aggregated based on a random-
effects model, using restricted maximum-likelihood (REML)
estimation to estimate the amount of heterogeneity in the
effect sizes.
21
We report the estimated average effect (^
m), the
estimated amount of heterogeneity in the true effects (^
t2),
and the estimated percentage of the total amount of
variability that can be attributed to heterogeneity (I
2
).
Corresponding 95% confidence intervals are given for all of
these statistics. We also provide the results from the test for
heterogeneity. The moderators were examined individually
and in combination using mixed-effects models, again using
REML estimation for the amount of (residual) heterogeneity.
For these models, we report the estimated effect sizes within
each level of a moderator with corresponding 95%
confidence intervals.
Results
Forty-four potentially relevant studies were found initially.
From these, 15 review articles were excluded immediately, as
these contributed no additional data besides the original
studies, which were already included. Another 13 studies
were excluded on the basis of title and abstract, as these
were studies conducted with animals. Five articles from
Asian authors were obtained through personal communi-
cation.
10–12,14,17
Four of them only had an English abstract
Effects of green tea on body composition
R Hursel et al
957
International Journal of Obesity
and were further written in Japanese. These articles were sent
to us with an English translation, as we contacted the
institution (Kao Corporation, Tokyo, Japan) where Nagao
et al.
13
are employed, after finding their study in PubMed.
Information from the fifth Asian study by Wang et al.
17
was
also obtained through personal communication, as the
results were not published during the data abstraction. Of
the remaining 21 studies, 9 did not use green tea (e.g.,
oolong tea) or did not provide sufficient information to
compute effect sizes. Only one study was excluded because
of the time period, as it lasted for 8 weeks. No studies
were excluded because of lack of randomization or blinding.
Finally, 11 articles fulfilled the inclusion criteria.
10–20
Figure 1
shows the process of study selection.
The selected articles all had comparable designs: in the WL
studies,
10–18
participants immediately started to consume an
EGCG–caffeine mixture for 12 weeks following randomiza-
tion; in the WM studies,
19,20
subjects first lost weight for
4 weeks using a very low energy diet, followed by randomi-
zation and then an EGCG–caffeine mixture supplementation
for either 12 or 13 weeks (Table 1). In all but one study,
17
subjects were randomly assigned to two conditions, with
seven studies
10,15–20
using a true placebo group and the
remaining four studies
11–14
comparing a high with a low
dose of an EGCG–caffeine mixture.
Almost every article reported the relevant information
about the moderators, for instance as an important inclusion
criterion. If regular caffeine intake was not reported by the
Asian studies, it was assumed to be low.
22
From three studies it was possible to calculate more than
one effect size estimate. Kozuma et al.
10
reported their results
separately for men and women, which, therefore, yields two
effect size estimates for the meta-analysis. Also, the data
from Westerterp-Plantenga et al.
20
allowed the calculation of
two estimates, one for subjects with high and the other for
subjects with low regular caffeine intake. Finally, three effect
size estimates were obtained from the study by Wang et al.,
17
as it contained three treatment groups that received either a
different dosage or an equivalent dosage distributed in a
different number of servings. A total of 15 effect size
estimates could, therefore, be extracted from the 11 studies.
Figure 2 shows a plot of the individual effect size estimates
with corresponding 95% confidence intervals.
The random-effects model indicated a modest, but sig-
nificant positive effect of catechins on WL/WM (^
m¼1.31,
95% CI: 2.05 to 0.57; Po0.001). Therefore, it is estimated
that subjects in the treatment groups lost on average
1.31 kg more weight (or gained on average 1.31 kg less
weight) than people in the control groups. Removal of any
individual study from the meta-analysis did not alter this
conclusion (with ^
mvalues ranging from 1.07 to 1.47; all
Po0.004). However, the results were clearly hetero-
geneous (^
m¼1.13 with 95% CI: 0.25 to 3.80; I
2
¼94% with
95% CI: 77% to 98.0%; test for heterogeneity: Po0.001).
The difference in dosage of the EGCG–caffeine mixture
between the treatment and the placebo group was not a
significant moderator, whether examined in combination
with the other two moderators or individually (P¼0.34 and
0.76, respectively). Although the average effect was larger for
studies with Asian (^
m¼1.51, 95% CI: 2.37 to 0.65) than
Caucasian (^
m¼0.82, 95% CI: 2.13 to 0.50) samples, the
difference did not reach statistical significance (P¼0.19
when examined in combination with the other two
moderators and P¼0.37 when examined individually).
The estimated effect was larger for samples with a low
(^
m¼1.60, 95% CI: 2.38 to 0.83) in comparison to a
moderate-to-high regular caffeine intake (^
m¼0.27, 95% CI:
1.63 to 1.10). The difference was significant when
examined in combination with the other moderator, but
not when examined individually (P¼0.04 and P¼0.09,
respectively). The estimated amount of (residual) hetero-
geneity was equal to ^
t2¼0.92 after inclusion of this
moderator in the model, suggesting that B19% of the total
amount of heterogeneity could be accounted for based on
information about the regular caffeine intake of the samples
in the set of studies.
Discussion
The results of this meta-analysis show that an EGCG–
caffeine mixture has an effect on WL and on WM after a
period of negative energy balance. It is also shown that
interaction of ethnicity and habitual caffeine intake is a
44 articles were found in
PubMed (N=44)
49 studies were included
(N=49)
11 articles were included
5 Articles obtained via
personal communication
(N=5)
23 articles excluded
Review studies (N=15)
Not sufficient
information (N=5)
Not green tea (N=4)
Duration of study too
short (N=1)
36 studies were included
(N=36)
13 articles excluded on
basis of title or abstract
(N=13)
Figure 1 Process of study selection.
Effects of green tea on body composition
R Hursel et al
958
International Journal of Obesity
moderator. Dosage of catechins in the treatment or in the
placebo did not have any significant effect on the outcome
of the studies that were included.
However, the overall result implies some heterogeneity.
This may be caused by studies differing in the possible
moderators ethnicity and habitual caffeine intake. In
particular, all Asian studies had a low caffeine intake and
nearly all Caucasian studies had a high caffeine intake. Only
one treatment group of the Westerterp-Plantenga et al.
20
study had a low caffeine intake. Although caffeine intake was
a significant moderator when examined together in a model
including ethnicity, inclusion of more studies with both
ethnicities with different amounts of caffeine intake would
provide a clearer picture of how these moderators influence
the effectiveness of catechins.
Interaction between the possible moderators maybe due to
differences in allele frequencies of polymorphisms between
ethnicities. For instance, people who drink caffeine regularly
not only get habituated to the effects of caffeine, but also
tolerate caffeine intake. Cornelis et al.
23
showed that
Hispanic Americans with an ADORA2A 1083TT genotype
are more likely to limit their caffeine intake than the CC and
CT genotype. These persons appeared to be more sensitive
for the differences between caffeine and the adenosinergic
system. This genotype codes for the A
2a
receptor, at which
caffeine antagonizes the actions of adenosine, a key process
in the stimulating properties of caffeine. CYP1A2, which
codes for the enzymes of cytochrome P450 (the metabolizers
of caffeine), does not seem to have a function.
23
Therefore, it
may well be that the habitually low caffeine consumers do
not tolerate high amounts of caffeine intake and, therefore,
react more sensitively to the small amount of caffeine added
to the green tea mixture. When the allele frequencies of the
genes coding for the activity of the enzyme differ between
ethnic groups, this may explain the observed differences in
response to the treatment. For this reason, ethnicity was
included as a moderator. Although ethnicity was not a
significant moderator, the results suggest that it may affect
the effect an EGCG–caffeine mixture has on thermogenesis
as the Asian subjects lost more weight than the Caucasian
subjects. However, before the moderating role of ethnicity
can be examined in more detail, additional studies with
Caucasian samples should be conducted to offset the larger
number of studies with Asian samples.
The inhibition of several enzymes may be the mechanism
by which the components of green tea induce WL/WM.
Catechins inhibit the catechol O-methyltransferase (COMT)
and caffeine inhibits phosphodiesterase. Levels of norepine-
phrine and cAMP rise because degradation by these enzymes
is ceased. Consequently, parasymphatic activity is in-
creased.
8
This phenomenon does not appear equally clear
in all ethnic groups because of the differences in relevant
enzyme activity, causing differences in sensitivity for these
ingredients. In that respect, Hodgson et al.
24
stated that there
is a wide variability in flavonoid O-methylation, a major
pathway of flavonoid metabolism, by the enzyme COMT.
Table 1 Characteristics of all included articles
Author Year Duration
(weeks)
Sample
Size (n)
BMI
(kg/m
2
)
Gender Age
(years)
Dosage
(mg per day)
Regular caffeine
intake (mg per day)
Ethnicity Outcome
Kovacs et al.
19
2004 13 GT ¼51 PL ¼53 25–35 M/F 18–60 GT ¼573 PL ¼04300 Caucasian No weight maintenance
Westerterp-Plantenga et al.
20
2005 13 GT ¼19 PL ¼19 25–35 M/F 18–60 GT ¼270 PL ¼0o300 Caucasian Weight maintenance
Westerterp-Plantenga et al.
20
2005 13 GT ¼19 PL ¼19 25–35 M/F 18–60 GT ¼270 PL ¼04300 Caucasian No weight maintenance
Diepvens et al.
18
2005 13 GT ¼23 PL ¼23 25–31 F 19–57 GT ¼1207 PL ¼04300 Caucasian No weight loss
Hase et al.
14
2001 12 GT ¼12 PL ¼11 18.5–30 M 27–47 GT ¼483 PL ¼119 o300 Japanese Weight loss
Nagao et al.
12
2001 12 GT ¼13 PL ¼12 24.3–34.6 M 24–49 GT ¼541 PL ¼130 o300 Japanese Weight loss
Tsuchida et al.
11
2002 12 GT ¼39 PL ¼41 24–30 M/F 30–65 GT ¼588 PL ¼126 o300 Japanese Weight loss
Kozuma et al.
10
2005 12 GT ¼53 PL ¼64 25–35 F 20–65 GT ¼540 PL ¼0o300 Japanese Weight loss
Kozuma et al.
10
2005 12 GT ¼54 PL ¼55 25–35 M 20–65 GT ¼540 PL ¼0o300 Japanese Weight loss
Nagao et al.
13
2007 12 GT ¼123 PL ¼117 24–30 M/F 25–55 GT ¼583 PL ¼96 o300 Japanese Weight loss
Auvichayapat et al.
15
2008 12 GT ¼30 PL ¼30 425 M/F 40–60 GT ¼750 PL ¼0o300 Thai Weight loss
Hsu et al.
16
2008 12 GT ¼41 PL ¼37 427 F 16–60 GT ¼491 PL ¼0o300 Taiwanese No weight loss
Wang et al.
17
2008 12 GT ¼49 PL ¼43 X24 o35 M/F 18–50 GT ¼440 PL ¼0o300 Chinese No weight loss
Wang et al.
17
2008 12 GT ¼47 PL ¼43 X24 o35 M/F 18–50 GT ¼468 PL ¼0o300 Chinese No weight loss
Wang et al.
17
2008 12 GT ¼43 PL ¼43 X24 o35 M/F 18–50 GT ¼880 PL ¼0o300 Chinese Weight loss
Abbreviations: BMI, body mass index; F, female; GT, EGCG–caffeine mixture group; M, male; PL, placebo group.
Effects of green tea on body composition
R Hursel et al
959
International Journal of Obesity
The inter-individual variability of the activity of COMT
could vary as much as three-fold.
24
Moreover, COMT enzyme
activity differs between ethnic groups,
25
in that Asian
populations have a higher frequency of the thermostable,
high activity enzyme, COMT
H
allele (Val/Val polymorphism)
than the Caucasian populations. The Caucasian populations
have a higher frequency of the thermolabile, low activity
enzyme, COMT
L
allele (Met/Met polymorphism).
3
In all, 50%
of Caucasians are homozygous for the COMT
L
allele (25%)
and COMT
H
allele (25%). The other 50% is heterozygous
(Val/Met polymorphism).
25
This may explain the difference
in sensitivity to interventions with EGCG–caffeine mixtures,
and why, in some studies with Caucasian subjects, no effect
was seen after ingestion of an EGCG–caffeine mixture. The
observations described above suggest interaction between
the two possible moderators. Not until the two moderators
were taken together, the difference in estimated effect
between samples with low and moderate-to-high regular
caffeine intake became significant, probably because of
coincidence of polymorphisms ADORA2A, COMT, and
ethnicity.
The results of the meta-analysis also show that the
difference in dosage of the catechins in the EGCG–caffeine
mixture between treatment and control groups had no effect
on the size of the effect. This finding is consistent with the
results from the study of Berube-Parent et al.
9
in which four
different mixtures of EGCG and caffeine with a different
dosage of catechins were examined. The results from that
study showed that all mixtures increased 24-h energy
expenditure and that the increase was similar for all doses
of EGCG in the mixtures, which was also found in the study
from Wang et al.
17
Taken together, this meta-analysis has shown that EGCG–
caffeine mixtures have a positive effect on WL and on WM.
-8 -6 -4 -2 0 2 4
Effect Size (in kg)
Kovacs, 2004
Diepvens, 2005
Westerterp-Plantenga, 2005
Westerterp-Plantenga, 2005
Nagao, 2007
Hase, 2001
Nagao, 2001
Auvichayapat, 2008
Tsuchida, 2002
Kozuma, 2005
Kozuma, 2005
Hsu, 2008
Wang, 2008
Wang, 2008
Wang, 2008
Caucasian, caffeine = 1
Caucasian, caffeine = 1
Caucasian, caffeine = 1
Caucasian, caffeine = 0
Asian, caffeine = 0
Asian, caffeine = 0
Asian, caffeine = 0
Asian, caffeine = 0
Asian, caffeine = 0
Asian, caffeine = 0
Asian, caffeine = 0
Asian, caffeine = 0
Asian, caffeine = 0
Asian, caffeine = 0
Asian, caffeine = 0
Overall Effect from
Random-Effects Model
Figure 2 Forest plot of the individual effect size estimates (mean weight change within the treatment group minus mean weight change within the control group)
with corresponding 95% confidence intervals (caffeine ¼0/1 for low (0) or moderate-to-high (1) regular caffeine intake).
Effects of green tea on body composition
R Hursel et al
960
International Journal of Obesity
Moreover, it also shows that habitual caffeine intake and
ethnicity may be moderators, especially because of inter-
actions of the different ADORA2A and COMT polymor-
phisms and ethnicity, whereas different doses of catechins
do not have a significantly different effect. Experimental
research is needed to find out whether and to what
extent people are genetically predisposed for the effect of
EGCG–caffeine mixtures.
Conflict of interest
The authors declare no conflict of interest.
Acknowledgements
RH and MSW-P designed the study. RH collected and
prepared the data. WV analyzed the data. RH and WV wrote
the manuscript and MSW-P contributed to the interpretation
of the data and reviewed the manuscript. The study was
executed under the supervision of MSW-P.
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